1. Field of the Invention
The present invention relates to memory devices which accumulate charge transferred from conductive layers in quantum dots provided therein.
2. Description of the Related Art
A typical memory device, such as an electrically erasable and programmable read only memory (EEPROM) and a flash memory, has a charge-accumulating layer surrounded by an insulating film, such as a silicon dioxide film, between a gate electrode and a conductive layer of a metal-oxide-semiconductor (MOS) transistor. When a high voltage is applied between a source electrode and a drain electrode and to the gate electrode in the memory device, charges, for example, electrons or holes, migrate in the insulating film by tunneling effects and are accumulated in the charge-accumulating layer. The amount of the accumulated charge reflects the quality of information. When the stored information is read out, a current flowing from the source electrode to the drain electrodes changes in response to the amount of charge accumulated in the charge-accumulating layer.
In conventional memory devices, the amount of charge accumulated in the charge-accumulating layer is controlled by the application time of the gate voltage. In this method, however, it is difficult to stably control the amount of accumulated charge, and a feedback circuit must be provided for the control. Furthermore, the charge must have a high energy so that the charge can clear the energy barrier of the insulating film. Thus, a high voltage is applied between the source electrode and the drain electrode and to the gate electrode. Accordingly, conventional memory devices, which do not work at low voltages, require large-scale integrated circuits, even when a simplified neuron circuit is formed.